Detecting that something isn't right and there might be a reason for it.
This is generally used in a situation of doubt or when spying comes into picture.This is applied when one has doubt over others actions.
1)"I smell something fishy",said the detective in an investigation.
2)"Robert, don't you smell something fishy going on here?",spied Barbera.
3)When Jack confessed his mistake,I smelled something fishy.